1. Field of the Invention
The present invention relates to a semiconductor integrated circuit device with a built-in photosensor (referred to as "IC with a built-in photosensor" hereinafter), and more particularly relates to techniques for improving electric characteristics of the IC with a built-in photosensor.
2. Description of the Background Art
A frequency of an analogue signal which is employed in an IC for a compact disk player to detect a light signal has been about 5 MHz. Recently, however, a request for processing a high-frequency signal of 20 MHz is increasing, particularly in an IC for a laser disk. Consequently, it is necessary to improve frequency characteristics of an IC with a built-in photosensor to a high-frequency signal. A conventional IC device with a built-in photosensor will be described with reference to FIG. 8. FIG. 8 is a plan view of a chip of an IC with a built-in photosensor employed in an IC for an optical pickup of a compact disk player and the like.
In the figure, numeral 1 denotes a light shielding aluminum film, numeral 2 denotes a pad for connecting a lead wire, numerals 3, 4 denote photosensors for detecting light. In the conventional IC device with a built-in photosensor, the whole chip except the photosensors 3, 4 for sensing light and the pad 2 is shielded from light with the light shielding aluminum film 1 to prevent faulty operation of a circuit because of an incident light.
Next, an outline of part of an integrated circuit for processing light signals detected by the photosensor 3 of the IC device with a built-in photosensor shown in FIG. 8 will be described with reference to FIG. 9. FIG. 9 is a block diagram showing a structure of an integrated circuit for processing a signal detected by a photosensor. In the figure, numeral 5 denotes a wiring, one end of which is connected to an output end of the photosensor 3, for transmitting a signal detected by the photosensor 3, numeral 6 denotes a capacitor, which is connected to the other end of the wiring 5, and a signal detected by the photosensor 3 is applied thereto, numeral 7 denotes a current-voltage converter circuit, an input end of which is connected to the wiring 5, for converting a signal detected by the photosensor 3 into a voltage, numeral 8 denotes a wiring for transmitting a signal passing through the capacitor, numeral 9 denotes a high-frequency amplifier for processing a high-frequency signal detected by the photosensor 3, numeral 10 denotes a wiring for transmitting a signal from the photosensor 4 and B1 through B8 denote signal processing blocks for processing an output signal from the photosensors 3, 4; B1 through B4 denote blocks for detecting a signal outputted from the photosensor 3, B5 denotes a high pass filter which passes only high-frequency signals among signals detected by the photosensor 3, B6 denotes a block for processing a high-frequency signal and B7 and B8 denote blocks for detecting if the photosensor 4 senses light. Accordingly, a high-frequency signal outputted from the photosensor is inputted to the high-frequency amplifier 9 passing through the wiring 5, the capacitor 6 and the wiring 8.
FIG. 10 is an enlarged view of the photosensor 3 and its vicinities. In FIG. 10, numeral 11 denotes a pad for connecting the photosensor 3 and the wiring 5. FIG. 11 is a perspective view taken along the lines C--C of FIG. 10 observed in the direction of the arrows. In FIG. 11, numeral 1 denotes a conductive light shielding film, numeral 15 denotes a layer insulating film and k1 denotes a peripheral region of the wiring 5. Other identical numerals with FIG. 8 indicate the same or corresponding parts of FIG. 8. As shown in the figure, parasitic capacity is generated between the wiring 5 and the aluminum light shielding film 1 in the region k1.
Generally, an overlapping portion of conductive layers, such as the conductive light shielding film 1 and the wiring 5 with the layer insulating film 15 in between shown in FIG. 12, can be regarded as equivalent to a parallel-plate capacitor consisting of two parallel conductive plates 20 shown in FIG. 13. Assuming the symbol C represents capacity of this capacitor, the symbol .epsilon..sub.r the relative dielectric constant of a substance in between two flat plates 20, the symbol d the distance between two flat plates 20, the symbol S the area of the flat plate 20 and the symbol .epsilon..sub.O the dielectric constant of vacuum, the following relation holds: EQU C=.epsilon..sub.O .multidot..epsilon..sub.r .multidot.S/d (1)
Accordingly, when the area wherein parasitic capacity can be generated becomes larger by employing a relatively large capacitor, long wiring or the like, parasitic capacity increases, so that the influence becomes great.
Next, FIG. 14 is a sectional view of a capacitor formed on a substrate. In the figure, numeral 21 denotes an aluminum electrode, numeral 22 denotes an N type semiconductor diffused layer, numeral 23 denotes a P type semiconductor diffused layer, numeral 24 denotes a nitride film, numeral 25 denotes an epitaxial growth layer and other identical numerals with FIG. 11 indicate the same or corresponding parts of FIG. 11. Parasitic capacity is generated at a region k2 where the aluminum electrode 21 and the light shielding aluminum film 1 are facing each other in this capacitor.
Since the conventional semiconductor integrated circuit device is structured as mentioned above, the light shielding aluminum film 1 covering over the whole chip generates parasitic capacity between itself and an element such as a wiring or a capacitor formed thereunder, as shown in FIG. 12. Particularly, since the high-frequency amplifier 9 formed in the IC is an amplifier for a light signal picked up by the photosensor 3 and the input impedance of the high-frequency amplifier 9 is high-impedance, there is the disadvantage that circuit characteristics such as frequency characteristics or the like are greatly deteriorated by the parasitic capacity in a line connected to an input end of the high-frequency amplifier 9.